Conductive Adhesive Versus Soldering for High Frequency Boards

Soldering attaches a component by forming an alloy with the pad. Conductive adhesive attaches it by suspending metal particles in a polymer that cures around the joint. The two mechanisms produce joints with different resistances, different temperature limits and very different process requirements, and the choice between them is usually forced by something other than electrical performance.

High frequency boards are one of the places where the question comes up, because the寄生 properties of a solder joint and of an adhesive joint differ, and because the temperature of soldering may be unacceptable for the substrate or for an adjacent component. Understanding where each method wins is the key to making the choice deliberately.

How Each Joint Conducts

A soldered joint is a metallurgical bond. The solder wets the pad and the component termination, and an intermetallic layer forms at the interface. Current passes from the pad through the intermetallic layer, through the bulk solder and through the second intermetallic layer. The resistance is low, typically a fraction of a milliohm for a small joint, and the joint is stable over a wide temperature range up to the melting point of the alloy.

An adhesive joint conducts through physical contact between particles and between the particles and the two surfaces. The polymer itself is an insulator, so conduction depends on the particle loading and on the contact pressure that the cured polymer maintains. The resistance is higher than a soldered joint, sometimes by a factor of ten or more, and it is sensitive to the cure schedule and to the thermal expansion of the polymer.

Temperature Limits and Process Windows

Solder requires a temperature above its melting point, which for a lead free alloy means a peak above 240 degrees Celsius. That temperature is applied to the whole assembly during reflow, or locally during repair, and it limits the components and the substrate that can be used. Where a board carries a component that cannot survive reflow, or where the substrate is a temperature sensitive material, soldering the whole assembly is not an option.

Conductive adhesive cures at 120 to 180 degrees Celsius, and some formulations cure at lower temperatures with a longer time. That is the principal advantage of the technology for temperature sensitive assemblies. The trade is that the cure is a chemical reaction that has to be completed, and an under cured joint has both a higher resistance and a lower mechanical strength. The cure schedule is therefore part of the electrical specification, not just a process detail.

High Frequency Behaviour

For a radio frequency connection, the difference between the two methods is not only the direct current resistance. A soldered joint presents a continuous metallic path with a well defined geometry, and its behaviour at high frequency is predictable from that geometry. An adhesive joint has a granular contact whose impedance has both a resistive and an inductive component, and the effective path is less well defined.

At the frequencies used on most boards the difference is small compared with the impedance of the transmission line, and it is not usually the deciding factor. Where the joint is part of a matching structure or a resonant element, the uncertainty in the adhesive joint becomes significant, and it is one of the reasons a solder joint is preferred for critical microwave connections. The same reasoning applies to the pad geometry, as discussed in the notes on pad design.

Mechanical and Thermal Cycling Behaviour

Solder is ductile and creeps under thermal cycling, which accommodates the expansion mismatch between a ceramic component and an organic board. The joint absorbs strain by deforming, and a properly designed joint lasts for thousands of cycles. Its failure mode is a crack that grows from the interface, and it can be detected by the rise in resistance that follows.

Adhesive joints behave differently. The polymer is elastic and can accommodate a limited amount of strain, but it is not ductile, and repeated cycling causes the contact between particles to degrade rather than the joint to crack. The resistance rises gradually rather than abruptly, which makes a trend measurement more informative than a pass and fail test. Adhesive joints also absorb moisture, and a humid environment raises both the resistance and the dielectric loss of the joint.

Where Each Method Is Used

Soldering remains the default for high volume assembly because it is fast, well understood and produces the lowest resistance joint. It is used wherever the assembly can tolerate the temperature and the components can survive the profile. Reflow in a conveyor oven, selective soldering and laser soldering are all variations of the same principle.

Conductive adhesive is used where the temperature is the constraint: on substrates that degrade, on assemblies containing a heat sensitive device, or where a low temperature process is needed for a specific component. It is also used for connections that must be made after reflow, such as attaching a shield or a flexible jumper, and for joints where the flexibility of the polymer is an advantage, as in some flex to board interconnections. In many products both methods appear on the same board, each used where it fits.

Practical Selection Criteria

The decision usually comes down to four questions. What is the lowest temperature the assembly can tolerate? What resistance is acceptable at the joint? What is the thermal cycling requirement over the product life? And how will the joint be inspected? When the answers favour adhesive, the design should also specify the pad finish, the particle loading, the cure schedule and the inspection method, because all four are part of making the joint reliable rather than merely making it.

It is worth noting that a process that is chosen for the wrong reason tends to produce a design that is difficult to manufacture. Selecting an adhesive to avoid a temperature problem, for example, is sound; selecting it to avoid investing in a reflow oven usually leads to a joint that is inspected by eye and fails in the field. Being explicit about the reason makes the design defensible and makes the process easier to control.

Process Control and Verification

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

FAQ

Can a conductive adhesive joint carry power? Only modest currents. The resistance and the current density limits of the polymer make it unsuitable for high current joints, which are soldered, welded or mechanically fastened.

Does an adhesive joint need flux? No, and that is one of its advantages. There is no flux residue to clean, which matters on assemblies where cleaning is difficult or impossible.

How is an adhesive joint inspected? By measuring the resistance of a test structure rather than by visual inspection, since the joint looks the same whether or not it is electrically sound.